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Absorbed Photosynthetically Active Radiation and Sun-View Geometry Effects on Remote Sensing Relationships

机译：光合作用吸收的辐射和太阳视几何关系对遥感关系的影响

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Quantifying the amount of photosynthetically active radiation (PAR) absorbed by vegetation is an essential consideration for determining useful vegetative photosynthetic capacity and surface conductance values for regional and global carbon cycle studies. This study was conducted to compare absorbed photosynthetically active radiation at the FIFE-89 Konza prairie sites to that of the KUREX-91 steppe grassland sites and to investigate variations in relationships between absorbed PAR and spectral vegetation indices derived from bidirectional reflectance factors. Incoming, reflected and transmitted PAR were measured from which fractions of reflected, transmitted and absorbed PAR were computed at selected FIFE prairie and KUREX steppe sites. Fractions of direct and diffuse PAR transmitted through canopies were estimated. Fractions of absorbed PAR were much lower at the FIFE sites (ranging from 0.35 to 0.65) than those at KUREX (ranging from 0.75 to 0.95) which can be explained by differences between leaf area index, leaf angle distribution, and direct and diffuse sky conditions. Scattering of PAR may be an important parameter in canopy light penetration particularly in canopies of large LAI, with non-photosynthetically active vegetation components and illuminated at large solar zenith angles. The magnitude of spectral vegetation indices computed and plotted as a function of the fraction absorbed did not differ considerably even though LAI and the fraction of absorbed PAR did. Adjusting for background improved the distinction between spectral vegetation indices at the two experimental sites. Relationships between fraction of absorbed PAR and spectral vegetation indices derived from bidirectional reflectance factors, were not consistent over illumination and view angles.
机译：量化植被吸收的光合有效辐射（PAR）的量是确定区域和全球碳循环研究中有用的营养光合能力和表面电导值的重要考虑因素。这项研究的目的是比较FIFE-89 Konza草原站点和KUREX-91草原草地站点吸收的光合作用辐射，并研究吸收的PAR与从双向反射因子得出的光谱植被指数之间的关系变化。测量传入，反射和透射的PAR，从中选择出的FIFE草原和KUREX草原位置计算出反射，透射和吸收的PAR的比例。估计通过冠层传播的直接和扩散PAR的分数。在FIFE部位吸收的PAR分数（从0.35到0.65）要比在KUREX吸收的PAR的分数（从0.75到0.95）低得多，这可以用叶面积指数，叶片角度分布以及直接和漫天条件之间的差异来解释。 。 PAR的散射可能是机盖光穿透的重要参数，尤其是在大的LAI机盖中，具有非光合活性的植被成分，并以大的太阳天顶角照明。尽管LAI和吸收的PAR的分数确实不同，但作为吸收分数的函数计算和绘制的光谱植被指数的幅度并没有太大差异。调整背景改善了两个实验点的光谱植被指数之间的区别。在照明和视角范围内，吸收的PAR的比例与源自双向反射因子的光谱植被指数之间的关系不一致。

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